Have you obtained a copy of his obituary?Being a school adminstrator, he probably not only had an obit, but possibly a news article about his passing.Sometimes family and some personal history is included in such.You didn't give any dates, or approximate dates, for David, I am assuming this is your man.
From TX Dth Idx Online 1964-1998:
David Kelly Sellars, d 26 Feb 1967 Tarrant Co, TX; single [can also mean widowed, separated or divorced]
TX Birth Idx 1926-1949:
Sellars, David Hallam, b 10 Mar 1926 Tarrant Co, TX Sellars, female infant, b 19 Jun 1929 Tarrant Co, TX Sellars, Josephine Ann, b 28 Sep 1931 Tarrant Co, TX
Mother -- Josephine Ross Father -- David Kelly Sellars
I did not find Josephine, she may have died too early to be on indexes.Ernest F. Sellars is on dth idx and SSDI.